In the case of domestic use, a prestressing strand is usually made by cold-drawing wires satisfying Japanese Industrial Standard (JIS) G 3502, stranding the wires, and giving a bluing treatment in order to remove residual strain thereof in the final step of the production. A prestressing strand satisfying the JIS G 3536 standard is usually used. In the case of non-domestic use, a prestressing strand is made from wires satisfying an appropriate standard (for example prEN10138), and the prestressing strand satisfying the appropriate standard is used. If strengths of those prestressing strands can be increased, the flexibility of structural design and the strength of a concrete construction using the strands will also be enhanced. That is, a prestressing strand having a decreased diameter can be used and an insertion pitch of the prestressing strands can be increased.
For example, in Patent document 1, a prestressing strand is proposed that is composed of 19 stranded wires and has a standard diameter of 19.3 mm, as specified in JIS G 3536. A tensile load of the prestressing strand (PC strand) significantly exceeds the lower limit specified in the standard, while the proposed prestressing strand has the same strand configuration and external diameter as a strand specified in the standard and a total cross-sectional area substantially equal to the nominal cross-sectional area specified in the standard.
[Patent document 1] Japanese Patent No. 3684186
However, when the tensile load applied to the prestressing strand described in Patent document 1 is significantly increased, it is extremely difficult to achieve a 0.2% permanent elongation load value and a relaxation value based on which the strand would be classified as a low relaxation strand as specified in the JIS G 3536 standards, while preventing the toughness from being deteriorated by strain aging during a wire drawing process. Moreover, a proposed prestressing strand having such a low 0.2% permanent elongation load value is not so suitable for practical use, and judging from the situations in which prestressing strands have been used in recent concrete constructions, there is little existence value for such a prestressing strand that cannot exhibit the desired relaxation value for a low-relaxation product.